The present invention relates to a nonaqueous electrolyte secondary battery, and more particularly to its separator.
Recently, for reduction of size and weight and extension of working time of notebook type personal computer, PDA, mobile telephone, camcorder, and other electronic appliances, development and improvement of secondary battery of high energy density are strongly demanded as built-in power source. As one of battery systems satisfying such demand, a high expectation is aimed at a nonaqueous electrolyte secondary battery of 4 V class using a lithium contained transition metal oxide such as LiCo2, LiNiO2 and LiMn2 O4 as the positive electrode, a carbon material such as graphite as the negative electrode, and an organic electrolyte composed of organic solvent solution of lithium salt as the nonaqueous electrolyte. This battery system is generally known as lithium-ion secondary battery, in which charging causes lithium ions to be deintercalated from the positive electrode, and intercalated to the negative electrode through the electrolyte. Likewise, discharging causes the lithium ions to be deintercalated from the negative electrode, and intercalated to the positive electrode through the electrolyte. Reactions of intercalation and deintercalation of lithium ions by such charging and discharging can be repeated reversibly, and a long cycle life is expected. In the lithium secondary battery using metal lithium in the negative electrode hitherto studied as secondary battery of high voltage and high energy density, metal lithium precipitates on the negative electrode and the lithium concentration in the electrolyte drops by charging, but from the terminal stage of charging to the overcharged region, dendrite metal lithium crystals precipitate and penetrate through the separator to cause internal short-circuit, and as a result not only the battery is unusable, but also there was a risk of accident of fire or rupture due to sudden increase of cell temperature. In the lithium-ion secondary battery, so far as it is not overcharged, dendrite metal lithium crystals do not precipitate, and the safety and reliability is enhanced very much. In this lithium-ion secondary battery, however, when charged at low temperature, diffusion of lithium ions is insufficient, dendrite metal lithium crystals may precipitate on the negative electrode, and internal short-circuit may be caused.
The separator for lithium secondary battery and lithium-ion secondary battery is made of a microporous thin membrane composed of at least one of polyethylene (PE) and polypropylene (PP) being insoluble in organic solvent such as ether and ester in organic electrolyte which is a nonaqueous electrolyte, sufficient in permeation of electrolyte, and easy in diffusion of lithium ions. More specifically, multilayer hybrid membranes of PE membrane and PP membrane or double layers or more of PE and PP with porosity of 40 to 70% in a thickness of 20 to 50 xcexcm have been used.
On the other hand, as means for meeting the demand of high energy density by high capacity of battery, it is important to fill the cell of specified overall dimensions with as many positive and negative active substances as possible. It is hence attempted to form separator interposed between the negative and positive electrodes as thinly as possible. This attempt is a contradictory means to the necessity of using a relatively thick separator from the viewpoint of preventing-internal short-circuit mentioned above.
It is hence an object of the invention a nonaqueous electrolyte secondary battery, using a relatively thin separator than in the prior art made of a microporous polyolefin group membrane of PE of a greater heat of fusion or multilayer of PE and PP, enhanced in reliability, without sacrificing the cell capacity, by effectively absorbing heat generation by reaction between the battery active substance and organic solvent in the electrolyte occurring at high temperature, in spite of a relatively thin separator, thereby suppressing the cell temperature rise, suppressing elevation of cell internal pressure, preventing electrolyte leak, and enhancing the resistance to electrolyte leak at high temperature.
The present invention relates to a nonaqueous electrolyte secondary battery, especially lithium-ion secondary battery, using a polyolefin group of endothermic calorie per unit area by the heat of fusion of at least 0.07 cal/cm2 and thickness of 15 to 30 xcexcm, preferably made of PE or multilayer of PE and PP, as hybrid separator. By using such separator, without sacrificing the cell capacity, internal short-circuit and temperature rise due to reaction heat between the battery active substance and organic solvent in the electrolyte at high temperature can be suppressed, gas generation is suppressed, and electrolyte leak due to elevation of cell internal pressure is eliminated, so that a high reliability is a ssured.